The advantages of plating chromium from an equilibrated aqueous solution of chromium (III) - thiocyanate complexes over conventional chromic acid plating are elaborated in our U.K. Pat. No. 1431639. Refinements and modifications of this basic process have been described in later patents among which are U.S. Pat. Nos. 4,141,803 and 4,161,432. The benefits to the trivalent chromium process of an anolyte and catholyte separated by a cation exchange membrane are described in pending U.K. patent application No. 13458/78. Finally pending U.K. patent applications Nos. 44177/78 and 7932300 describe a related solution and process in which beneficial effects are obtained from a reduction in the level of chromium and thiocyanate concentration to levels well below those originally contemplated by those skilled in the art.
Equilibrated chromium (III) - thiocyanate complexes from which plating takes place have been prepared from a variety of starting materials. The originally preferred starting salts of U.K. Pat. No. 1431639 were chromium perchlorate and sodium thiocyanate. In order to make the solution sufficiently electrically conductive additional sodium perchlorate was added as a supporting electrolyte. U.S. Pat. No. 4,141,803 proposed hexathiocyanatochromium salts of potassium or sodium (K.sub.3 Cr(NCS).sub.6 or Na.sub.3 Cr(NCS).sub.6) to which sodium perchlorate or sodium sulphate was added as a conductivity salt. Potassium sulphate was also mentioned as a possible conductivity salt but no example was given. In U.S. Pat. No. 4,161,432 one preferred solution was prepared from chromium chloride (CrCl.sub.3) and sodium thiocyanate. Potassium chloride was added for conductivity. A second preferred solution was prepared from chromium sulphate (Cr.sub.2 (SO.sub.4).sub.3) and sodium thiocyanate. In this case sodium sulphate was added for conductivity.
In pending U.K. application No. 13458/78, in which a catholyte and anolyte are separated by a membrane, the catholyte was prepared from chromium sulphate (Cr.sub.2 (SO.sub.4).sub.3) and sodium thiocyanate, and sodium chloride was added for conductivity. The anolyte consisted of an aqueous solution of a depolarising agent to which sodium sulphate (Na.sub.2 SO.sub.4) was added for conductivity. The advantage of having sodium sulphate in the anolyte rather than sodium chloride is that chlorine evolution from the anode is very much reduced. The electrolyte employed in pending U.K. application No. 7932300 has essentially similar constituents to that of U.K. application No. 13458/78 except that the concentration of chromium is below 0.03 molar and the concentration of thiocyanate is also proportionally reduced.
It is found that in plating chromium from electrolytes as described in U.K. application Nos. 13458/78 and 7932300, with catholyte and anolyte separated by a cation exchange membrane, chloride ions from the catholyte are, in practice, able to penetrate the membrane is sufficient numbers to give significant chlorine evolution at the anode. This is not only environmentally undesirable but prevents the use of cheap lead anodes because of formation of lead chloride thereon. Instead, platinized titanium anodes have had to be used. A further problem with baths having chloride anions in the catholyte is that pH stability is poor and needs frequent adjustment.